The goal of this project is to develop single fiber optical sensors for pH and CO2 based on polymer swelling. In Phase I, Polysense has demonstrated the feasibility of this concept by preparing a pH sensor consisting of a small amount of aminated polystyrene on the tip of a single optical fiber. Protonation of the amine causes the polymer to swell due to electrostatic repulsion. This is accompanied by an increase in the clarity of the polymer and a decrease in the amount of light reflected back into the fiber. The resulting sensor was approximately 150 micrometers in diameter and underwent hundreds of swelling and shrinking cycles without degradation. It responded to pH in the range of physiological interest. The mechanism of response appears to involve changes in pore size as the polymer swells.The goals of Phase II include: improving the performance of the sensor by using new approaches to polymer formulation, evaluating how polymer properties depend on formulation, evaluating sensor response as a function of polymer formulation. preparing a CO2 sensor, developing a convenient calibration procedure, and evaluating the performance of the pH sensor in invasive pH measurements in animals.